This invention relates to circuits and methods for harmonic neutralization of voltage waveforms in static inverters and, more particularly, to such circuits and methods which reduce or eliminate harmonic components from an output waveform by controlling the phase angle of various waveforms and combining such waveforms in a transformer circuit.
Static inverters employing switching devices are commonly used for many applications including DC/AC power conversion, variable speed drives, back-up power supplies and generation of controllable reactive power for utility transmission and distribution systems.
Presently, differences in the switching characteristics of semiconductor devices at various power levels, make it advantageous to employ different techniques to fabricate the voltage in low, medium and high power systems. For low power applications, very fast switching devices (such as MOSFETs) and construction techniques which minimize voltage spikes are practical. In low power systems, high frequency switching techniques and small filters are used to produce a high quality output voltage waveform. For medium power applications, lower switching frequencies and somewhat larger filters are usually employed. In medium power systems, devices generally switch somewhat slower and have well defined limits on the maximum di/dt and dv/dt that they can withstand. These devices therefore require di/dt and dv/dt limiting circuits and have a limit on their maximum switching frequency. For very high power systems, where the largest switching devices are required, presently available devices are only suitable for low frequency operation. In high power systems, it is common practice to employ special power circuit configurations which combine outputs of a number of stages each operating at the lowest possible frequency.
For very large systems where efficiency is important, and many switches are required to attain the power level, it is advantageous to operate the switches at fundamental frequency and employ harmonic neutralizing techniques to obtain a high quality output waveform. Classical harmonic neutralized inverters (HNIs), utilize multiple sets of phase displaced fundamental frequency square waves combined with special phase shifting transformers to realize high quality multi-step output waveforms. Each step of the output waveform is evenly spaced and has an amplitude proportional to the sine of its angular position. The number of steps is referred to as the pulse number.
In this type of multi-step inverter, each switching device operates with identical voltage and current waveforms and contributes equally to the output. Because all switches turn on and off at the same levels of current, all devices operate with similar delays and the effects of differences in current dependent switching delays upon the output waveforms are minimized. The harmonic spectrum of the synthesized output contains terms having harmonic H.sub.n orders of: EQU H.sub.n =np.+-.1
and relative amplitude A.sub.n : EQU A.sub.n =1/(np.+-.1)
where n is the pulse number and p is any integer. If a high quality waveform is desired, a high pulse number is clearly an advantage.
The basic building block of all of the relevant prior art systems is the six-pulse inverter bridge. A six-pulse bridge includes three inverter poles connected across a DC voltage source. Each pole has two switching devices connected in series, the junction of the switching devices being the AC output terminal. The inverter poles each operate at fundamental frequency and produce three square wave outputs with respect to the mid point of the DC voltage. The three outputs are symmetrically displaced by 120 degrees so that a pole transition occurs every 60 degrees or in other words, there are six state changes in a cycle of fundamental frequency.
The output voltages produced between the three AC terminals have true six pulse waveforms. The six-pulse bridge inverter forms the basic building block generally used to make up all higher pulse number harmonic neutralized inverters. To produce true harmonic neutralized outputs having pulse numbers of N.times.6, the outputs of N six-pulse bridges are combined as follows. The bridges are operated from a common DC source with their outputs incrementally phase displaced by an angle which corresponds to one segment of the desired multi-segment or multi-pulse output, that is, a displacement angle of 360/6N degrees. The fundamental outputs of individual six-pulse bridges are shifted into phase with each other by individual transformers having appropriate winding configurations and the same voltage ratios. The transformed outputs of each bridge, each having the same fundamental amplitude and phase, are combined either by the series connection of the secondaries, by parallel connection through appropriate interphase transformers, or by some combination of series and parallel connections.
To obtain the required phase shift between the primary and secondary of the transformer connected to each six-pulse bridge, different winding configurations are used on each transformer. While the improvement in waveform quality obtainable with higher pulse numbers is significant, the number of fractionally rated special transformers increases, and the cost cannot be justified for most applications. Therefore, simple transformers are preferred and 12-pulse configurations employing wye/wye and delta/wye windings are most common.
Techniques that combine the outputs of six-pulse bridge inverters to produce greatly improved "quasi harmonic neutralized inverter" (QHNI) outputs with less complicated transformer configurations are illustrated in U.S. Pat. No. 4,870,557, which is hereby incorporated by reference. A particularly advantageous configuration for a QHNI employs two transformers having open-wye/open-wye and open-wye/closed-delta windings fed from two pairs of six-pulse inverters. The first pair of inverters are operated at phase angles which lead the fundamental output by 7.5 degrees and 172.5 degrees. The second pair of inverters are operated at phase angles which lead the fundamental output by 37.5 and 202.5 degrees. The resultant fundamental voltages impressed across the primaries of the two transformers are phase displaced by 30 degrees so that the secondary voltages are in phase and add directly to produce a 24-pulse output.
To prevent zero sequence current form circulating in the delta winding of the second transformer, a zero sequence blocking transformer (ZSBT) having three identical windings on a single core, is connected in series with the open wye primary of the wye/delta transformer. Fundamental voltages produced at the secondaries are both in-phase at zero degrees and sum directly to form the resultant 24-pulse output. While the harmonic spectrum of the voltage waveform produced by this arrangement meets the requirements for many applications, for an advanced static VAR generator connected to high voltage transmission lines, it is desirable to reduce the amplitudes of the residual low order (11th and 13th) harmonics somewhat further.